Since having characteristics of being energy saving, lightweight, thin, and the like, liquid crystal display devices have been spread rapidly in recent years in place of conventional CRT displays.
Since such a liquid crystal display device has a polarizing element in the image display face side of a liquid crystal cell and generally, it is required to give hardness to the polarizing element for protecting the polarizing element from scratches at the time of handling, the image display face is commonly provided with hardness by using a hard coat film obtained by forming a hard coat layer on an optically transparent substrate as a polarizer protection film.
Conventionally, a film made of a cellulose ester typified by triacetyl cellulose has been employed as the optically transparent substrate of the hard coat film. It is based on advantageous properties of the cellulose ester: that is, as being excellent in transparency and optical isotropy and scarcely having in-plane phase difference (low retardation value), the cellulose ester extremely scarcely changes the vibration direction of incident linear polarization, scarcely affects on the display quality of a liquid crystal display device, has proper water permeability, and accordingly can dry out water remaining in a polarizing element through an optical layered body when the polarizer using the optical layered body is produced.
However, a cellulose ester film is a material disadvantageous in terms of the cost and is insufficient in moisture resistance and heat resistance and when a hard coat film is used as a protection film for a polarizer in high temperature and high humidity environments, the cellulose ester film adversely deteriorates the polarization function and polarizer function such as color phase.
Because of these problems of the cellulose ester film, it has been tried to use a polyester substrate of polyethylene terephthalate or the like which is excellent in transparency, heat resistance, and mechanical strength and a material economical as compared with the cellulose ester film, as a material for an optically transparent substrate in place of the cellulose ester film.
However, because having aromatic rings with high polarizability in the molecular chain, the polyethylene terephthalate film has extremely high intrinsic birefringence and tends to exhibit in-plane birefringence in the film along with orientation of the molecular chain by stretching treatment carried out for providing excellent transparency, heat resistance, and mechanical strength. Therefore, in the case where the polyethylene terephthalate film is put on a polarizing element, there is a problem that the liquid crystal display device shows nonuniformity with different colors (hereinafter, referred to also as “rainbow interference pattern”) particularly when the display screen is viewed obliquely and the display quality of the liquid crystal display device is deteriorated.
As a trial of using a polyester substrate as a material for an optical substrate in place of a cellulose ester film, for example, Patent Literature 1 discloses a polarizer protection film, which is a film containing a polyester resin as a main component and made to have an in-plane retardation Re of 500 nm or higher. In the invention disclosed in Patent Literature 1, in order to provide the polyester film with sufficient mechanical strength, biaxial drawing is carried out at vertical and transverse draw ratio of 3.3/3.9 and the retardation is thus inevitably generated, and since the draw ratio is low and the vertical and transverse draw ratios are almost equal, the retardation value is at minimum 500 nm and at maximum 700 nm. However, with retardation of such a low level, the rainbow interference pattern problem cannot be solved. In the invention disclosed in Patent Literature 1, the rainbow interference pattern problem is solved by forming a light diffusion layer with a haze of 10 to 80% on the uppermost layer. Although the rainbow interference pattern can be solved by formation of the light diffusion layer having a haze of 10% or higher, there occurs another problem of image quality deterioration such as white muddiness or contrast.
For example, Patent Literature 2 discloses an antiglare film using a polyethylene terephthalate film drawn 2.5 to 6 times and having sufficient transparency as a transparent substrate. Regarding this antiglare film, if the retardation is 1000 or higher, the coloring is not noticeable in the front but the color nonuniformity (rainbow interference pattern) in an oblique direction cannot be solved and therefore, the rainbow interference pattern is solved by making the total haze at least 8 times as high as transmission clarity. However, if the transmission clarity is lower, the visibility is lowered so that the antiglare film disclosed in Patent Literature 2 is required to have a haze of 5.5 to 55%. Further, in order to satisfy the relationship between the transmission clarity and the haze, the specular reflectivity of an antiglare layer is made so extremely low as 0.05 to 2% by increasing the period of uneven forms on the surface of the antiglare layer and therefore, the antiglare film has few flat face and the rainbow interference pattern can be solved; however, it causes a problem of image quality deterioration such as white muddiness or contrast.
Patent Literature 3 discloses that good visibility regardless of the viewing angle can be attained when a screen is viewed through a polarizer such as sunglasses by using a white light-emitting diode as a light source and using and arranging a polymer film with a retardation of 3000 to 30000 nm in a manner that the angle formed between the absorption axis of the polarizer and the slow axis of the polymer film is at 45 degree. However, a polyester film or a polycarbonate film, which is a preferable polymer film in Patent Literature 3, is soft and has no scratching resistance and therefore, the polymer film cannot stand practical use unless a hard coat layer is formed on the surface of the polymer film. In the case where a hard coat layer is formed on the surface of the polymer film, if a refractive index difference between both becomes wide, interference fringes attributed to the refractive index difference are generated, resulting in image quality deterioration.
The interference fringes mean a phenomenon that when white light comes to a transparent thin film, the light reflected by the front face of the thin film and the light once incident on the thin film and reflected by the rear face thereafter are interfered with each other and seen like a partial rainbow color-like hue, and a phenomenon caused by change of mutually intensified wavelength depending on the viewing direction. This phenomenon is not only uncomfortable but also unpleasant for a user in some cases, and improvement is strongly required. In the case where a hard coat layer (refractive index: Nh) is formed on a polymer film (refractive index: Np) and Np and Nh are different (refractive index difference), for example, in the case where Np is 1.64 to 1.68 and Nh is 1.50 to 1.53, reflected light interference is caused in the interface of the polymer film and hard coat layer and the interference fringes are more significant as the refractive index difference is wider.
On the other hand, it has been known that the interference fringes can be solved by making the refractive indexes of the polymer film (refractive index: Np) and hard coat layer (refractive index: Nh) as even as possible (hereinafter, also referred to as interference fringe solution method 1). Further, an intermediate layer is formed between the polymer film and the hard coat layer (e.g., a primer layer for adhesiveness improvement) in some cases, and in this case, there is also a known technique of suppressing the interference fringes by adjusting the refractive index of the intermediate layer to a middle refractive index between the refractive index (Np) of the polymer film and the refractive index (Nh) of the hard coat layer (hereinafter, also referred to as interference fringe solution method 2) (e.g., see Patent Literatures 4 and 5). The middle refractive index (Nph) between the refractive index (Np) of the polymer film and the refractive index (Nh) of the hard coat layer can be calculated theoretically according to the following mathematical expression.Nph=√{square root over (Np·Nh)}
That is, in the invention disclosed in Patent Literature 3, it is necessary to form a hard coat layer based on the above-mentioned interference fringe solution method 1 and to form an intermediate layer based on the above-mentioned interference fringe solution method 2 for interference fringe prevention.